This invention relates generally to perfluorinated polyethers and uses thereof. More particularly, the invention pertains to metal salts of perfluorinated polyethers having carboxylic acid end groups, to methods for synthesizing the metal salts, and to use thereof as anti-wetting and/or corrosion-protective agents, e.g., in anti-wetting and/or corrosion-protective coatings on metal-containing substrates, particularly in magnetic recording devices such as magnetic recording disks and magnetic recording heads.
It is well known that organic carboxylic acids, R-COOH, in a fluid state (liquid or vapor) primarily exist in the hydrogen bonded dimeric form, as follows: 
It is also well known that the vapor phase in equilibrium with solid NaCl at elevated temperature (Txe2x89xa7500xc2x0 C.) is dominated by the dimeric species 
The formation and stability of the latter are attributed to the pair-wise Coulombic interaction of the constituent ions; see Doan et al. (1997) J. Am. Chem. Soc. 119:9810. The heat of dimerization for the acetic acid dimer has been measured to be 15 kcal/mol (Taylor (1951) J. Am. Chem. Soc. 73:315; Weltner (1955) J. Am. Chem. Soc. 77:3941), and that for (NaCl)2 to be 48 kcal/mol. The corresponding pair-wise interaction between salt molecules of carboxylic acids, e.g., sodium acetate, has only recently been reported, by Doan et al. (1997), supra.
Perfluoropolyethers (PFPEs) are currently in use as lubricants in a variety of high-performance applications. PFPEs are commercially available in several distinct structural forms. Representative PFPEs are known by the brand names Demnum(copyright) (Daikin Kogyo Co., Ltd., Japan), Krytox(copyright) (DuPont Specialty Chemicals, Deepwater, N.J.), and Fomblin(copyright) Z (Zentek SRL, Milan, Italy), having the following structural formulae 
Krytox is synthesized by base-catalyzed polymerization of hexafluoropropylene oxide, as described by Gumbrecht (1966) ASLE Trans. 9:24, while Demnum is made similarly but starting with 2,2,3,3-tetrafluorooxetane. The hydrogen atoms in the resulting polymers are replaced by fluorine atoms by subsequent contact with F2 in solution, as described by Ohsaka (1985) Petrotech (Tokyo) 8:840. Fomblin Z is synthesized by photooxidation of tetrafluoroethylene and is a linear, random copolymer of ethylene oxide and methylene oxide units; see Sianesi (1973) Chim. Ind. 55:208.
These PFPEs are also available with carboxylic acid end groups, as exemplified by Fomblin(copyright) Z-DIAC (Zentek SRL, Milan, Italy), Krytox(copyright)-H (DuPont) and Demnum(copyright)-SH (Daikin Kogyo Co. Ltd., Japan), having the structures 
respectively. It has been found that the sodium salts of these polymers normally exist in the dimeric form under ambient conditions; see Doan et al. (1997), cited supra.
The inventors herein have now discovered that these and other metal salts of perfluorinated polyethers having one or more carboxylic acid groups are extremely effective anti-wetting agents and thus find utility in a host of applications, for example in corrosion-protective films. Although Doan et al. describes a method for synthesizing sodium salts of PFPE acids, the method described is problematic. That is, Doan et al. describes preparation of sodium salts of PFPE acids by reacting a PFPE acid (e.g., Fomblin(copyright) Z-DIAC, Krytox-H(copyright) or Demnum(copyright)-SH) with a sodium hydroxide solution, and then extracting the salt from the resulting emulsion with a fluorocarbon solvent. Although the intended product may be prepared using this technique, the method requires handling a multilayer fluid including a viscous interfacial gel layer, a cumbersome process that requires extreme care. This invention is in part directed to a new method for synthesizing metal salts of perfluorinated polyether acids that overcomes the aforementioned disadvantage of the Doan et al. synthesis.
The invention is also premised on the discovery that metal salts of PFPE acids are useful as anti-wetting agents and corrosion-protective agents. In this regard, it should be pointed out that certain fluorinated polymers, particularly poly(fluoroalkylacrylates) and poly(fluoroalkylmethacrylates), have been used as oil and water repellent agents (see, for example, B. E. Smart, xe2x80x9cOrganic Fluorine Compoundsxe2x80x9d in Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Vol. 10, John Wiley and Sons, New York, 1980, p. 869). However, the adhesion force obtained using these polymers as coating agents is insufficient to provide sufficient durability in many contexts.
One important application of the present compounds that exploits the newly discovered properties is as corrosion-protective agents that bond strongly to metal and metal oxide substrates, as the compounds adhere well to metal-containing substrates. Furthermore, the compounds of the invention are extremely useful as corrosion-protective agents for magnetic recording disks and magnetic recording heads, particularly those having a carbon overcoat. Such overcoats are typically formed by sputter deposition from a graphite target, and are generally called protective carbon overcoats, xe2x80x9cdiamondlikexe2x80x9d carbon overcoats, amorphous carbon overcoats, or, in the case of those overcoats formed by sputter deposition in the presence of a hydrogen-containing gas, hydrogenated carbon overcoats. Tsai et at. in xe2x80x9cStructure and Properties of Sputtered Carbon Overcoats on Rigid Magnetic Media Disks,xe2x80x9d J. Vac. Science Technology A6(4), July/August 1988, pp. 2307-2314, describe such protective carbon overcoats and refer to them as amorphous xe2x80x9cdiamondlikexe2x80x9d carbon films, the xe2x80x9cdiamondlikexe2x80x9d referring to their hardness rather than their crystalline structure. IBM""s U.S. Pat. No. 4,778,582 describes a protective hydrogenated disk carbon overcoat formed by sputtering a graphite target in the presence of Ar and hydrogen (H2). The carbon overcoats may also be formed by plasma-enhanced chemical vapor deposition (CVD) and may include nitrogen in addition to hydrogen, as described by Kaufman et al. (1989) Phys. Rev. B 39:13053.
To increase the areal density of the data magnetically recorded on the disk, the recording head must be brought close to the magnetic layer, which means that the overcoat thickness must be substantially reduced, i.e., to less than 5 nm in future disk drives. Consequently, an important challenge faced by the disk drive industry is how to make protective disk overcoats that are ultra-thin yet still provide the desired durability and corrosion protection. However, the carbon overcoat sputter-deposited on the magnetic layer of storage disks often abounds with pinholes, through which the corrosion of metals in the magnetic and other underlayers may occur. Reducing the thickness of the carbon overcoat exacerbates the problem. The same drawback is encountered with the metallic elements of magnetic recording heads that are coated with a layer of sputtered carbon. Because metal salts of PFPE acids adhere well to metal-containing surfaces and poorly to carbon overcoats, the compounds are able to fill pinholes in the protective overcoat without adding any substantial thickness to the disk. Anti-corrosion strength is further enhanced by the exceptional water repellency of PFPE acid salts.
Accordingly, it is a primary object of the invention to address the above-mentioned need in the art by providing a method for synthesizing metal salts of PFPE acids.
It is another object of the invention to provide such a method which involves treating a perfluorinated polyether having at least one carboxylic acid group with a metal salt of a volatile organic acid, and then volatilizing the resulting organic acid.
It is another object of the invention to provide a method for treating a metal-containing substrate to enhance water repellency and corrosion resistance, wherein the method involves depositing a composition containing a metal salt of a perfluorinated polyether acid onto the substrate.
It is an additional object of the invention to provide such a method wherein the metal-containing substrate has a metal surface or a surface comprised of a metal oxide.
It is a further object of the invention to provide a corrosion-protected magnetic recording disk comprised of a substrate, a magnetic layer, and an amorphous carbon overcoat on the magnetic layer that has been treated with a composition containing a metal salt of a perfluorinated polyether acid as a corrosion-protective agent.
It is yet a further object of the invention to provide a corrosion-protected magnetic recording head having an amorphous carbon overcoat that has been treated with a composition containing a metal salt of a perfluorinated polyether acid.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.